Various electrochromic mirror and electrochromic window systems, collectively known as electrochromic elements, are generally known in the art. Such electrochromic elements typically change their light reflectance or transmittance properties in response to variations in environmental light conditions. For instance, a typical electrochromic rear view mirror normally operates in a full reflectance mode during the day, but reduces its reflectance at nighttime to protect the driver from glare effects from light emanating from the headlights of vehicles approaching from behind. Such automatic rear view mirrors have become increasingly sophisticated over the years, and the automatic rear view mirror for automotive vehicles disclosed in U.S. Pat. No. 4,443,057, issued Apr. 17, 1984, for Automatic Rearview Mirror for Automotive Vehicles, and assigned to the assignee of the present invention, is typical of such sophisticated automatic rear view mirrors. An improved electronic control system for automatic rear view mirrors is disclosed in U.S. Pat. No. 4,580,875, issued Apr. 8, 1986, for Electronic Control System for Automatic Rearview Mirrors for Automotive Vehicles, also assigned to the assignee of the present invention. The disclosures of U.S. Pat. Nos. 4,443,057 and 4,580,875 are specifically incorporated herein by reference.
Typically, these automatic rear view mirrors use an electrochromic chemical whose opacity changes in response to a voltage applied across it. One or more light sensors determine the amount of light in front of and/or behind the vehicle. Based on this information, a voltage is applied across the electrochromic chemical to cause it to become either more or less opaque. Thus, the automatic rear view mirror responds to glare from behind the vehicle by automatically shifting to a partial or low reflectance mode. After the source of the glare is removed, the automatic rear view mirror returns to the full or high reflectance mode without any driver intervention.
Electrochromic elements are also used, for example, in light-sensitive windows. In this application, an electrochromic chemical responds to an applied voltage by changing its opacity. As in the automatic rear view mirror, one or more light sensors detect the amount of light present. Based on the ambient light level, a voltage is applied across the electrochromic chemical, causing it to change its opacity. For example, during the day, the relatively high ambient light level causes one voltage level to be generated that causes the electrochromic chemical to become more opaque, making the window appear darker, thereby blocking potentially harmful sunlight, for example. By contrast, at night, the low ambient light level causes another voltage level to be generated that causes the electrochromic chemical to become less opaque. As a result, the window becomes more transparent. Of course the converse can be facilitated if more transparency is desired during daylight and more opacity is desired at nighttime.
In both of these types of electrochromic elements, the applied voltage level, or drive voltage, affects the operational characteristics of the electrochromic element. A high drive voltage, for example, optimizes the transition time for changes in opacity. The drive voltage involved in achieving acceptably fast transition times is somewhat higher than the voltage involved in minimizing reflectance in the steady state condition. High temperatures further increase the drive voltage required to cause the electrochromic element to darken fully to the center. The extra drive voltage overcomes losses attributable to series resistance in the transparent conductive coatings. As the drive voltage increases, however, the electrochromic chemical becomes more susceptible to degradation. Consequently, the color and functional characteristics of the chemical are adversely affected, and its life span is shortened. These effects are particularly pronounced at low temperatures, e.g., during the winter.
Accordingly, a relatively high drive voltage is desirable in high temperature environments, while a relatively low drive voltage is better suited for cold weather. Many conventional electrochromic elements employ a compromise drive voltage, trading off between full darkening in hot weather and element life at low temperatures. As a result, such electrochromic elements suffer from incomplete darkening and/or shortened element life to some degree.